There is a rising trend in the area of food products and beverages to provide at least part of the product in the form of foam. This raises technical challenges to provide foams that remain stable for a long time.
Foams are meta-stable systems of very low shelf life. The gas bubbles, which are dispersed in a liquid phase are stabilized by thin films of surfactants (e.g. milk proteins). As soon as a foam is formed, destabilization occurs due to drainage (liquid drains by gravity), coalescence and disproportionation (due to gas pressure differences between bubbles of unequal size).
Foams and emulsions are behaving in different ways, so that specific solutions have to be developed for the stabilization of foams or emulsions, respectively. Foams are thermodynamically more unstable and generally have a lifetime of some orders of magnitude smaller than emulsions (hours compared to months). The main reasons for this are that the films between bubbles are generally much larger than between emulsion droplets and the interfacial tension on the air-water interface is larger than on the oil-water interface by a factor of around 5. Also, bubbles are typically larger and less dense than oil droplets and therefore gravity creaming is much faster for foams than for oil in water emulsions. Foams are therefore prone to drainage, disproportionation (the equivalent of Ostwald ripening in emulsions) and coalescence. Additionally, when considering foams, the different instability mechanisms can reinforce each other by acting synergistically towards the collapse of the foam. For example, disproportionation results in larger bubbles and therefore larger films which in turn accelerate drainage, continuously decreasing the foam stability. Therefore, it is impossible to infer that an emulsifier successful to stabilize an emulsion would also be successful in stabilizing foam. The reverse is also true. Specific stabilizers have to be developed for foams.
Foam destabilization can be delayed by high viscosities of the liquid phase (like in a mousse), by drying the system (e.g. dough) or by freezing the system as in ice cream. However, it is still desirable to find solutions to the problem of stabilizing liquid foams.
It is known, that particles can stabilize foams due to the so called Pickering-effect. Numerous publications describe this effect in details. However there is still a need to identify suitable food-grade particles, which act as Pickering particles to stabilize food foams.
There is an additional trend to increase the stability of foams while trying not to introduce new ingredients to formulations. Especially in milk products, it is desired to improve the properties of such milk products, including foam stability, while using existing components of milk. The whey protein fraction of milk has been widely used in food formulations because of its high nutritional value but also for its broad functional properties. For example E. A. Foegeding, P. J. Luck, and J. P. Davis, Food Hydrocolloids, 2006, 20, 284-292 studies the ability of whey protein isolates (WPI) to stabilize foams.
H. Zhu and S. Damodaran, J. Agric. Food Chem., 1994, 42, 846-855 has shown that polymerisation of whey protein fractions by sulfhydryl-disulfide interactions improve the foaming properties. While proteins can form films with high interfacial elasticity and viscosity through the various forms of cross-linking of the adsorbed molecules on the interface, it was found that they still can't completely stop bubble shrinking and foam instability and there is therefore still a need to find new foam stabilizers. In addition this type of stabilization increases the viscosity and stiffness of the foam. It is desirable to find solutions to stabilize foam while retaining its liquid appearance.
A large number of products exist on the market, which are in powder form and are intended to be reconstituted by the consumer and to foam upon reconstitution, such as coffee and tea creamers. This is also true for powdered products intended to be reconstituted in vending or beverage preparation machines. In such a case, it would be very advantageous to find a foam stabilizer in the form of solid particles that could be incorporated in the powder product.
J. D. Firebaugh and C. R. Daubert, International Journal of Food Properties, 2005, 8, 243-253 teaches that dried derivatized whey protein isolates are capable of stabilizing foams at least as well as native whey protein isolates. In particular, it describes that derivatized whey protein isolates create foams that are more stable and have lower overrun than unmodified WPI foams when the pH of the foam is adjusted to pH 3.4 and 6.8. Derivatized when protein isolate is produced at pH 3.4.
The present invention addresses the above-mentioned problems by providing an efficient solution for the stabilization of foams.